Horrible fuel mileage after knock sensor replacement

[Fubaru]

97 impreza OBS 159k

Last time I scanned it, I got - knock sensor, cyl 1 MF, cyl 2 MF, evap(0440).

I installed a knock sensor, cleared the codes, and have driven about 150mi. No CEL so far and the car runs much smoother but I am still getting horrible fuel mileage. not just bad but like 12mpg BAD . Any ideas? THX.

[WoodsWagon]

Knock sensor failure won't cause misfire codes, so you probably still have an ignition system problem to work on. When was the last time plugs and wires were done?

 

Cyl's one and two share the same coil, so it could be a coil or ignitor failure, but start with the plugs and wires.

[idosubaru]

+1 to what he said - issue likely resides there.

 

when removing/installing the knock sensor it's not hard to damage the connector/have the pins pulled and not fully connecting. i would expect a check engine light and code of course but it is possible to have sensor issues without codes, it does happen occasionally.

[uniberp]

Short drives, stop and go, cold engine, cold air, cold grease, winter gas, I am not really surprised at 12 mpg.

 

If it were stinking gas, stumbling or rough, or slipping I would look to fixing those.

[davebugs]

+1 to what he said - issue likely resides there.

 

when removing/installing the knock sensor it's not hard to damage the connector/have the pins pulled and not fully connecting. i would expect a check engine light and code of course but it is possible to have sensor issues without codes, it does happen occasionally.

 

What GG said.

 

That should be a 2.2 which historically aren't picky about plug wires - ther than they need to be good.

 

But it's likely electrical and NOT related at all to knock sensos. Infact rarely does a knock sensor give a code, and when it does it is for knock sensor. So you likely went down the wrong path there.

 

But coil, wire, or plug are best suspects. Cheapest things to do at this point. Go get NGK plugs (I get the 2.00 ones with the green printing on the plug NOT the blue ones). Swap out plugs and while you're at it moves wires front to back.

 

If this doesn't solve it and the code stays for 1&2 it's the coil. If the problem moves to 3&4 it's the wires. So you've identified or cured the real problem for 10 bucks and some labor.

[Fubaru]

The only code that resurfaced since I changed the knock sensor is 0440 which I am aware of the possible issues with. I'll do a tune up and see what happens. Excuse me while I put my rubber arms on and go cuss some spark plugs.

[uniberp]

pull that airbox and washer reservoir and save yourself a bunch of aggravation